Various kinds of use application of inkjet printers for personal use, office use, various types of business use, and for document recording, for displaying color images, and for color photographs have been emerged due to their high functionalization. Furthermore, the use application has been extended to industrial use where high speed printing is required. The development of aqueous pigment-based inkjet inks has been advancing toward production of finer pigment particles in the ink for the purpose of improving the clearness, color vividness, color density, etc. of pigments. On the other hand, ejection droplets (ink droplets) are becoming finer associated with the development toward high speed printing and high image quality by improving devices. These improvements bring about outstandingly high image quality on processed paper, particularly photo paper and wide format printing paper, etc. for inkjet. However, the extension of the use application to the inkjet printing system in recent years is not limited to these kinds of paper, and, as listed below, a high printing adaptability to various kinds of paper quality (material quality) has been required, however, with the conventional technologies, a problem that printed characters or printed images (printed matter) are rubbed off when rubbed with a finger has occurred depending on the kind of paper or film.
For example, printing on films has been conducted as industrial use, however the maintenance of the adhesiveness of a printed coating film formed by printing is required at a high level. Moreover, with respect to the films used for printing, there exist films having a wide variety of material quality such as polyvinyl chloride-based films, polyolefin-based films, PET films, and polystyrene-based films, however the realization of the printed matter having a high adhesiveness to any one of those films has been required.
In order to solve the problems and requirements described above, it becomes necessary to add a binder for forming a printed coating film into the inks, and the following performance has been required for the binders. That is to say, it has been considered that the binder that is applicable to substrates having various types of paper quality, that is a binder for forming a coating film by which a high adhesiveness is achieved to film substrates having various types of material quality, and that makes an ink, when the binder is contained in the ink, favorable in terms of inkjet printability (high speed printability and ejection stability) is required. Moreover, it sometimes occurs that inks dry on a head or the like of a printer in inkjet inks, and it has been considered that the “redissolvability” by which, even when the inks dry in the head or the like, a colorant is dissolved or dispersed in water again with a cleaning liquid or the ink itself is required as a particular performance in inkjet inks. Hereinafter, paper sheets and films as a substrate for printing are sometimes referred together to as the “substrates”.
Facing the above-described requirements, various binders as an additive have been developed. For example, a water-soluble polymer solution or a polymer emulsion solution that can usually be a coating film component, such as an acryl-based, urethane-based, or vinyl-based water-soluble polymer solution or polymer emulsion solution, has been added to the inks. However, when the water-soluble polymers are used, the polymers dissolve in a water-based media in aqueous pigment-based inks, thereby making the viscosity of the inks high, and therefore there is a problem that the water-soluble polymers are not suitable for inkjet inks. Further, with respect to the water-soluble polymers, it sometimes occurs that a polymer which exhibits water-solubility by making an alkali neutralized product from the polymer having a carboxy group is used, however there exist the following problems. That is to say, in this case, the water-solubility can be made high and the redissolvability of the inks becomes favorable by increasing the concentration of carboxy groups, however since the polymers dissolve, the viscosity of the inks tends to become high, and, moreover, the inks exhibit non-Newtonian viscosity and therefore it sometimes occurs that the inks, when applied to inkjet printing, cannot be favorably ejected. Furthermore, since the amount of carboxy groups is large, there has been a problem that the water fastness of the formed coating film may sometimes be poor.
Moreover, on the contrary, the following advantages are obtained by using a water-soluble polymer in which the concentration of the carboxy groups is lowered, however, in this case, there occurs another problem. By lowering the concentration of the carboxy group, the solubility of the polymer disappears and the polymer becomes in a form of particles, thereby taking a form of aqueous dispersion or emulsion, and therefore the viscosity of inks can be lowered. Further, since the concentration of carboxy group is low, the water fastness of the printed matter becomes favorable, and since the polymer is hydrophobic, the adhesiveness with substrates is improved. However, since the water solubility of the polymer is poor, there is a problem that the redissolvability of inks by which the inks dissolve or disperse again in a water-based medium becomes poor when the inks dry. In order to solve this problem, a water-soluble solvent has separately been used in the conventional technologies.
On the other hand, examples of the polymer emulsion used for inkjet inks include an emulsion obtained by emulsion polymerization using a publicly known low molecular weight surface active agent or reactive surface active agent and a self-emulsifying type emulsion obtained by adding a small amount of monomer having a carboxy group, then subjecting the monomer to polymerization, and neutralizing the resultant polymer. However, since the polymer itself becomes a high molecular weight polymer to improve the adhesiveness to the substrates in these polymer emulsions, once the emulsions turn into a coating film by drying, the coating film does not dissolve in water-based media, therefore it sometimes occurs that the redispersibility becomes poor, and there has been a problem that the redissolvability of inks that is particularly important in inkjet inks cannot be realized (see, Patent Literatures 1 to 3).